Chapter%208%20Experiments

1
Chapter 8 Experiments

• Topics Appropriate to Experiments
• The Classical Experiment
• Selecting Subjects
• Variations on Experimental Designs
• An Illustration of Experimentation
• "Natural" Experiments
• Strengths and Weaknesses of the Experimental
  Method

2
Topics Appropriate to Experiments

• Projects with limited and well-defined concepts.
• Projects that are exploratory rather than
  descriptive.
• Studies of small group interaction.

3
Components of Experiments

• Three Pairs
• Independent and dependent variables
• Pretesting and posttesting
• Experimental and control groups

4
Experimental and Control Groups

• Must be as similar as possible.
• Control group represents what the experimental
  group would have been like had it not been
  exposed to the experimental stimulus.

5
Selecting Subjects

• Probability sampling
• Randomization
• Matching

6
Randomization and Matching

• May not know which variables will be relevant for
  matching process.
• Most statistics used to analyze results assume
  randomization.
• Randomization only makes sense if you have a
  large pool of subjects.

7
Preexperimental Research Designs

• One-shot case study - single group of subjects is
  measured on a variable following experimental
  stimulus.
• One-group pretest-posttest design - adds a
  pre-test for the group, but lacks a control
  group.
• Static-group comparison - includes experimental
  and control group, but no pre-test.

8
Sources of Internal Invalidity

• Historical events may occur during the course of
  the experiment.
• Maturation of the subjects.
• Testing and retesting can influence behavior.
• Instrumentation

9
Sources of Internal Invalidity

• Statistical regression of subjects starting out
  in extreme positions.
• Selection biases.
• Experimental mortality - subjects drop out of the
  study before it's completed.
• Demoralized control group subjects.

10
Limiting External Invalidity

• Solomon four-group design
• Posttest-only control group design

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Solomon Four-group Design

• Four groups of subjects, assigned randomly
• Groups 1 and 2 are the control and experimental
  group.
• Group 3 does not have the pre-test.
• Group 4 is only posttested.

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Posttest-only Control Group Design

• Includes Groups 3 and 4 of the Solomon design.
• With proper randomization, only these groups are
  needed to control the problems of internal
  invalidity and the interaction between testing
  and stimulus.

13
"Natural" Experiments

• Important social scientific experiments occur
  outside controlled settings and in the course of
  normal social events.
• Raise validity issues because researcher must
  take things as they occur.

14
Experimental Method

• Strengths
• Isolation of the experimental variable over time.
• Experiments can be replicated several times using
  different groups of subjects.

15
Experimental Method

• Weaknesses
• Artificiality of laboratory setting.
• Social processes that occur in a lab might not
  occur in a more natural social setting.